Zinc oxides are used in the manufacture of zinc halide brines. Although both premium and low grade zinc oxides can be used to produce the zinc halide brines, the low grade material can be more cost effective and environmentally beneficial because low grade zinc oxides are often recycled as byproducts from other industrial processes. These byproducts, however, may contain various impurities. The process of purifying the low grade zinc oxide is both expensive and time consuming. Using the recycled zinc oxides or zinc oxides containing impurities without the benefit of a purification process results in an end product, zinc halide for example, that has impurities.
Electrogalvanizing waste sludge, for example, has been used as a less expensive zinc source for producing zinc bromide and zinc chloride brines. The electrogalvanizing waste consists of primarily zinc hydroxide but can contain impurities such as iron, manganese, and nickel. These metal ions create problems in the final product by tinting the solution, and/or slowly precipitating. Iron and manganese can be removed from the zinc bromide and zinc chloride brines by oxidation, followed by filtration. The removal of nickel from solutions of zinc chloride is not a simple task. The general properties of nickel and zinc are similar enough that common separation methods such as pH precipitation are not usable. There is ample evidence in the mining literature that nickel can be removed by cementation onto elemental zinc dust in zinc sulfate solutions. The initial reaction favors reduction of nickel (II) to nickel, observed as the cementation of nickel onto the zinc surface. In turn the nickel metal and zinc metal can react with acidity in the solution to dissolve the metals. These reactions pose the following process considerations and production problems: excessive generation of hydrogen gas, increased zinc consumption, redissolution of nickel, and precipitation of zinc hydroxide or zinc oxide if too much acidity is consumed.
The evolution of hydrogen gas poses potential safety and engineering problems. Hydrogen is a flammable gas with a lower explosive limit of 4% and an upper explosive limit of 76.4%. This means that hydrogen, at concentrations of 4% to 76.4% in air, can ignite explosively. Also hydrogen can cause embrittlement of some metals. Therefore, lower levels of hydrogen gas generation are desired. The second process consideration is the increased consumption of zinc metal. The consumption of zinc by reaction with acid reduces the efficiency of the cementation process. Zinc consumed in this reaction results in the generation of zinc halide and hydrogen. The relative expense of consuming zinc in this reaction depends on the relative cost of the zinc oxide and the zinc metal used in the system.
The redissolution of nickel has been encountered during the batch removal of nickel from zinc chloride by zinc cementation. The nickel level in the zinc chloride is reduced through a minimum value followed by a subsequent rise in nickel concentration. This reduction of nickel concentration followed by an increase in nickel concentration has made the batch removal of nickel to a fixed concentration difficult.
A fluidized zinc bed used to remove impurities, specifically cadmium, from zinc sulfate solutions is taught in U.S. Pat. No. 3,954,452. A method of purifying zinc solutions in which zinc shot or scrap is employed to remove copper and cadmium is disclosed in U.S. Pat. No. 1,587,695. In the method of the '695 patent, zinc salt solutions containing salts of more negative metals are percolated upwardly through a compartment containing loosely packed particles of metallic zinc particles, then percolated in opposite directions through second and third compartments of greater areas, each containing metallic zinc particles.
U.S. Pat. No. 1,496,004, disclose a process for preparing pure zinc sulfate solutions in which zinc shot or balls (page 1, column 2, lines 85-86) are used to remove copper from neutral or nearly neutral solution. Arsenic, antimony and copper containing ores are leached with dilute sulfuric acid; ferric iron is precipitated from the solution; and copper is precipitated from the resulting solution by means of zinc oxide. The copper-bearing residue is further treated with sulfuric acid to dissolve the copper. The copper is precipitated out from the resulting solution.
U.S. Pat. No. 2,128,380, discloses a process for removing zinc values from concentrates. In the process, zinc dust is mixed with an acidified solution that contains zinc chloride, and the zinc dust precipitates cadmium (page 1, column 2, lines 30-49). U.S. Pat. No. 4,168,970, discloses a method for the purification of zinc sulfate solutions containing copper, cadmium and cobalt as major impurities; the method comprises the steps of 1) treating the impure zinc sulfate solution with zinc dust and an antimony and/or arsenic compound; 2) treating the partially purified solution with zinc dust and antimony and/or arsenic compound to form a cement containing essentially zinc as well as the remaining cobalt and other incidental impurities; and 3) recycling at least the major portion of the cement to be used instead of zinc dust during the first stage.
The problem of producing zinc brine from recycled zinc byproducts or zinc byproducts containing impurities has not been adequately resolved by prior teachings. It is desirable to have a method of producing zinc brine that removes nickel and other metal impurities without increased zinc consumption, redissolution of the nickel or excessive generation of hydrogen gas.